Page 142 - Inorganic Mass Spectrometry - Fundamentals and Applications

132 Olesik

Fire assay techniques with large (15-30 g) samples of crushed, ground rocks;
soils; and sediments are often used to concentrate precious metals such as Au, Pt,
and Pd while providing a more homogeneous sample. Appropriate assays using Pb
or NiS, for example, and their subsequent dissolution depend on the elements of
interest [293]. Reagent blank levels often limit detectable levels of platinum group
elements and gold. Chlorination methods have been used [294] in large part
because of the very low reagent blank levels attained. Acid dissolution and
do
extraction procedures have also been described, although acid extractions often
not yield quantitative extraction of the precious metals. Because acid extractions
are often more rapid and inexpensive, they are sometimes used to screen large
numbers of samples. Sodium peroxide fusions have also been used to form salts
that are water-soluble. Purification and preconcentration using precipitation, ion
exchange separation, and solvent extraction are then often used [294]. By using a
lead fire assay followed by cupellation and AgCl separation, detection lirnits for
Pt, Pd, and Au of 0.1 ng/g in soil and sediment have been attained [295].

Isotope Ratio and Isotope Dilution ~easure~ents
Isotope ratio precision provided by quadrupole ICP-MS ins~ments is much
poorer (typically 0.2% to 0.8% RSD) than can be obtained by thermal ionization
mass spectrometry (TIMS). Plasma fluctuations and short-term variations in the
aerosol introduced into the plasma are the main sources of noise in ICP-MS.
Multicollector sector-based ICP-MS ins~ments can provide isotope ratio preci-
sion of 0.003% to 0.05% RSD, in some cases rivaling that of TIMS [58]. Sector-
based instruments can also provide very high sensitivity compared to that of
TMS, p~icularly for elements with higher ionization energies those that form
or
refractory compounds. In these cases isotope ratio precision can be superior to
TIMS precision. Sample preparation for TIMS is generally much more time-
consuming and restrictive than for ICP-MS. Measurement times for ICP-MS are
generally shorter because of higher ion signals so that counting statistics are better
on a relative basis. Elements that form refractory compounds or have high
ionization energies are not amenable to TIMS. Multicollector sector-based ICP-
MS ins~ments can provide isotope ratio precision that rivals that of TMS [SS].
However, there are several applications in which the isotope ratio precision
provided by quadrupole or single-collector sector instruments is sufficient.
Os isotope ratios vary as a function of age as a result of radioactive decay of
ls7Re to Is7Os. Os is much more extensively ionized in the ICI? than by TIMS.
Precision provided by quadrupole ICP-MS (0.3% relative standard deviation) is
sufficient to gain useful dating by Os isotope ratio measurement [296,297]. Pb
isotope ratios vary, depending on the geological history of the sample, because of
decay of 238'u, 235U, and 232Th to 206Pb and Z07Pb.
Often high accuracy is required for geological interpretation of trace ele-